Light detection and ranging (LIDAR) systems have been developed that, in addition to range measurements, are capable of remotely measuring winds in connection with weather forecasting and climate studies, and relative velocities of hard targets in connection with target identification and threat assessment. Lidar systems have also been used in connection with remote sensing of chemical compounds, gasses, and aerosol optical properties in the atmosphere, and surface chemistry and physical properties of hard targets. In general lidar operates by transmitting light from a laser source to a volume or surface of interest and detecting the time of flight for the backscattered light to determine range to the scattering volume or surface.
A Doppler wind lidar additionally measures the Doppler shift experienced by photons scattered back to the instrument due to the motions of molecules and aerosols (e.g. particles and droplets) in the scattering volumes. The speed of the wind is determined from the line of sight (LOS) speed of the molecules and aerosols relative to the lidar. However, such systems have been limited to taking brief snapshots of wind speed over relatively small areas. In addition, the range of such systems has been limited, because of the small number of photons that are returned to a detector when ranges are large. As a result, lidar systems placed in low earth orbit (LEO) are relatively close to the surface and therefore travel at a significant speed relative to the surface of the Earth, limiting their ability to economically collect data with the spatial and temporal coverage needed for various environmental and defense applications.
The remote sensing of chemicals in the atmosphere can be accomplished using differential absorption lidar (DIAL). In a DIAL system, light at different wavelengths is transmitted across a volume of interest, and the strength of the backscatter intensity from subsequent volume or surface backscatter is used to determine the relative attenuation across the volume in order to identify those wavelengths that are absorbed by the compounds in that volume of interest and their relative concentrations. Accordingly, the system must be capable of operating at a number of different wavelengths. However, such frequency hopping has made simultaneous wind lidar measurements difficult or impossible with traditional Doppler Wind Lidar methods, particularly using a common, integrated instrument. As a result, measurements of wind have been made separately from chemistry measurements, resulting in chemical flux measurements that are highly susceptible to errors due to temporal and spatial sampling discrepancies. In addition, variations in the aerosol backscatter and extinction properties either spectrally or spatially within the measurement volume leads to additional errors in DIAL measurements.
Another technique for sensing the chemistry and other features, such as temperature and pressure, of volumes in the atmosphere or other target volumes include Raman lidar. However, as with DIAL systems, Raman lidar has only been possible using systems that are separate from a Doppler wind lidar. Accordingly, the difficulties with flux measurements mentioned above in connection with DIAL instruments have also been present in connection with HSRL and Raman lidar systems.